COMMUNITY ORGANIZATION: STRATIFICATION 



465 



hand, the elements of the succeeding vege- 

 tational stratum, or strata, are discontinu- 

 ous since each plant arises from the floor as 

 a unit. In forests, where there are several 

 vegetational strata, this structural discon- 

 tinuity is partially alleviated by the over- 

 lapping of foliage and the binding action of 

 vines. 



Soil plants and animals have been classi- 

 fied in a variety of ways. According to 

 their size, they may be divided into (a) 

 microbiota, including bacteria, algae, fungi, 

 protozoans, rotifers, larvae, small species of 

 nematodes, and minute mites; (b) meso- 

 biota, consisting of small forms just visible 

 with a hand lens up to animals several 

 centimeters long, and holding such groups 

 as Enchytraeidae, many nematodes, and 

 the majority of soil-inhabiting mites, crusta- 

 ceans, centipedes, millipedes, spiders, in- 

 sects, and other arthropods, as well as 

 snails; and (c) macrobiota, consisting of 

 plant roots, earthworms, and vertebrates, 

 such as toads, certain lizards, snakes, ro- 

 dents, moles, and other relatively large ani- 

 mals (cf. Fenton, 1947). 



As to community organization, the sub- 

 terranean and floor strata hold a variety of 

 organisms that may be classified according 

 to their food habits and their efiFects on the 

 soil. Such a system has been discussed and 

 revised by Fenton (1947), and this may 

 be further extended: (1) chemical agents, 

 such as bacteria and fungi, that are directly 

 and indirectlv involved in the chemical 

 conversion of humus; (2) ectophagous 

 agents (Jacot, 1939) that consume leaves 

 from without and in entirety. Such agents 

 include (Fenton, 1947) (a) species con- 

 cerned only with mechanical breakdown 

 and partial chemical conversion of the floor 

 litter, such as green leaf eaters (snails, 

 caterpillars, many chrysomelid beetles, 

 rodents, birds, deer), and animals feed- 

 ing upon the autumnal or seasonal fall 

 of leaves from higher strata and the floor 

 detritus (snails, mites, Collembola, and 

 certain larvae of flies); (b) species 

 concerned not only with subdivision of the 

 litter, but also with its incorporation into 

 the soil. Here belong the important lumbri- 

 cid worms, ants, millipedes, certain termites 

 in subtropical and tropical floors, and ro- 

 dents, particularly of grassland communi- 

 ties (Grinnell, 1923). (3) Entophagous 

 agents (endophagous animals of Jacot, 



1939, and Fenton, 1947) that feed inside 

 of litter units, and are divisible into (a) 

 leaf miners of fallen leaves (hoplodermatid 

 mites and the larvae of some sciarid flies); 

 (b) tunnelers of dead rootlets, providing a 

 system of minute channels down which may 

 be swept finely divided products of decom- 

 position (such as many tyroglyphid 

 and oribatid mites, and certain collem- 

 bolans and fly larvae). (4) Predators, 

 embracing (a) mesofauna which, by rea- 

 son of their relatively small size, make 

 Httle direct contribution to the mixing of 

 mineral and organic materials (centipedes, 

 spiders, carabid, staphylinid, and pselaphid 

 beetles); (b) macrofauna which, by rea- 

 son of their relatively large size and active 

 fossorial habits, make a larger contribution 

 to the mixing of mineral and organic ma- 

 terials (moles, certain rodents and shrews). 

 (5) Shelterers that use the floor and sub- 

 terranean strata for hibernation, aestiva- 

 tion, or as a protected niche in which to 

 pass a resting or developmental stage, or 

 for protection against attack by enemies. 

 This is a large and complex group, includ- 

 ing animals that make regular seasonal and 

 diel vertical movements into these lower 

 strata, as well as horizontal movements 

 from other communities. Furthermore, these 

 five categories of soil organisms are 

 modified by another classification that 

 places emphasis upon the amount of time 

 soil organisms spend in the subterranean 

 stratum and in the floor stratum. 



Burrowing and tubicolous marine poly- 

 chaetes are paralleled by burrowing ter- 

 restrial oligochaetes, not only in habitat 

 niche, but in having the orifices of their 

 burrows opening on the surface of the suc- 

 ceeding stratum. Among these annelids the 

 sand-swallowing and mud-swallowing ma- 

 rine species occupy an analogous feeding 

 niche with the soil-s^vallowing and debris- 

 swallowing earthworms. In a similar anal- 

 ogous position are such burrowing forms 

 as sea cucumbers, while the numerous bur- 

 rowing crustaceans occupy a feeding niche 

 analogous to that of the burrowing soil in- 

 sects. 



The subterranean stratum is well pro- 

 vided with fossorial vertebrates that live all 

 or a part of their lives within its bounda- 

 ries, together with numerous species that 

 excavate into this stratum from the floor 

 above. These include amphibians ^caeci- 



